1. Field of the Invention
The present invention mainly relates to a direct digital synthesizer, which can output a sine wave at a predetermined frequency.
2. Background Information
Conventionally, as described in FIG. 8(a), DDSs (Direct Digital Synthesizers) each including an NCO (Numerically Controlled Oscillator), a ROM, and an LPF (Low-pass Filter), have been known.
The NCO outputs a sawtooth wave at a predetermined frequency. The ROM stores, as a table, output values (a phase of the sawtooth wave) from the NCO and an amplitude (displacement) of a sine wave which is set corresponding to the output values. By performing a conversion based on this table, the sawtooth wave can be converted into the sine wave. Then, this sine wave is outputted after a high frequency element thereof is removed by an LPF.
JP2005-045674A (Patent Document 1) includes an integration circuit having a similar configuration to the NCO, a multiplication circuit, and an error correcting unit. The multiplication circuit performs predetermined operation on a sawtooth wave, which is to be outputted from the multiplication circuit, to convert this sawtooth wave into a parabolic signal. The error correcting unit corrects the parabolic signal obtained from the multiplication circuit, based on an ideal sine waveform stored in a storage in advance. Thus, a sine wave can be formed.